1. Field of the Invention
The invention is directed to a developer unit for an electrographic printer or copier with a toner acceptance surface accepting a layer containing toner particles, referred to in brief as toner layer.
2. Description of the Related Art
The structure of such a developer unit with a developer drum as toner acceptance surface is set forth in U.S. Pat. No. 4,777,106. For this developer unit, containing a toner reservoir where a toner-air mixture is located, and where toner particles are electrically charged and are subsequently deposited on the toner acceptance surface. This surface is either grounded or provided with a potential, and the charged toner particles are deposited thereon as a toner layer under the influence of an electrical force field. Due to the rotation of a developer drum whose generated surface forms the toner acceptance surface, the deposited toner particles are conducted past a development gap between the developer drum and a toner image carrier. The toner image carrier carries a latent charge image onto which toner particles are selectively applied at the development gap, creating a toner image. The toner image is then applied onto an ultimate image carrier, for example onto paper, from the toner image carrier with or without employing an intermediate image carrier.
A developer belt can also be used in place of the developer drum. A developer unit having more than one toner acceptance surface is explained in European Patent EP 0 494 454. The second toner acceptance surface is formed by the generated surface of a transfer drum on which the toner particles are applied directly from the reservoir. The toner particles are then deposited onto the surface of the developer drum at a transfer gap between the transfer drum and the developer drum.
The known developer units do not allow monitoring of the developing process and, in particular, of the amount of toner output by the developer unit. When the toner amount per surface on the toner acceptance surface or the charge amount per surface in the toner layer during developing lies outside of a predetermined range the error is not recognized until the finished print image is produced, resulting in a delayed corrective response. The disturbances in the print image are particularly noticeable for large-area picture elements.
Both dry and liquid toners can be used. The toners that are employed can also be divided into single-component and multi-component toners. Electrographic printers encompass electrophotographic printers, ionographic printers and magnetographic printers.
U.S. Pat. No. 5,006,897 or, respectively, U.S. Pat. No. 4,026,643 disclose developer units wherein the properties of a toner particle developer layer forming a magnetic brush are acquired with the assistance of a potential sensor and a capacitative circuit or, alternately an optical sensor.